1. Field of the Invention
The present invention relates to an image forming apparatus having a developing unit for developing a latent image formed on a latent image carrier. More particularly, the present invention relates to an image forming apparatus capable of effecting control to prevent adhesion of a developer to an uncharged region of a latent image carrier.
2. Description of the Related Art
Image forming apparatuses, such as a copying machine, a printer, and a facsimile, use a latent image forming type recording apparatus, e.g., an electrophotographic apparatus, to meet the demand for plain paper recording. In such an image forming apparatus, an electrostatic latent image is formed on a photosensitive drum and then developed to a visible image with a toner. After the toner image has been transferred to a sheet of paper, the sheet is separated from the photosensitive drum, and then the toner image on the sheet is fixed thereto.
In the above-described image forming apparatus, the presence of unnecessary toner on the photosensitive drum, which is not transferred to the sheet, leads to unnecessary consumption of the developer, thus giving rise to problems in terms of both environment and cost. In cleanerless image forming apparatuses, in which no cleaner for the photosensitive drum is provided, but the residual toner on the photosensitive drum is collected at the developing unit, the amount of unnecessary toner to be collected increases. Consequently, uncollectable toner locally remains as a residual image on the photosensitive drum, which produces an adverse effect on the following image formation. Thus, there has been a demand for a technique which eliminates the presence of unnecessary toner on the photosensitive drum.
FIGS. 1A and 1B illustrate a conventional image forming apparatus. As shown in FIG. 1A, an electrophotographic printer has a photosensitive drum 1 of, for example, an organic photosensitive material, a Se photosensitive material, an a-Si photosensitive material, etc. Around the photosensitive drum 1 are disposed a corona charger 2 for uniformly charging the surface of the photosensitive drum 1, a laser optical system 3 for effecting image exposure, a developing unit 4, e.g., two-component developing unit, a magnetic, single-component developing unit, non-magnetic, single-component developing unit, etc., a roller transfer unit 5 for electrostatically transferring the toner image formed on the photosensitive drum 1 to a sheet of paper 8, and a cleaner 6, e.g., a fur brush cleaner, a blade cleaner, etc. In addition, a fixing unit 9 is disposed on a transport path for feeding the sheet 8. The fixing unit 9 fixes the toner image to the sheet 8 by heat or pressure.
The printing operation of the image forming apparatus is as follows: First, the surface of the photosensitive drum 1 is uniformly charged by the corona charger 2. Next, the charged surface of the photosensitive drum 1 is exposed to a light image corresponding to an image to be printed by the laser optical system 3, thereby forming an electrostatic latent image in accordance with the image. Next, as the photosensitive drum 1 passes the developing unit 4, a precharged toner adheres to the electrostatic latent image on the photosensitive drum 1. Thus, a toner image is formed.
Meantime, the sheet 8 to be printed with the image is fed to a position where it comes in contact with the toner image formed on the photosensitive drum 1. The roller transfer unit 5 presses against the photosensitive drum 1 under a predetermined pressure with the sheet 8 held therebetween and charges the sheet 8 in opposite polarity relation to the toner charge. Thus, the toner image on the photosensitive drum 1 is electrostatically transferred to the sheet 8. While the sheet 8 carrying the toner image is passing through the fixing unit 9, the toner image is fixed to the sheet 8 by heat and pressure.
In the meantime, the toner remaining on the photosensitive drum I after the toner image has been transferred to the sheet 8 is removed and collected by the cleaner 6. Thus, the photosensitive drum 1 is returned to the initial state to repeat the printing operation.
The toner removed and collected from the photosensitive drum 1 by the cleaner 6 is transported by a toner transport mechanism (not shown) to a waste toner tank (not shown) where it is temporarily stored. When a predetermined amount of waste toner has been collected, it is taken out of the apparatus and thrown away by the user.
In the above-described image forming apparatus, the electric charge accumulated on the photosensitive drum 1 disappears during the time when the apparatus is at rest or stands by for a print command. Particularly, in a region of the photosensitive drum 1 that is defined between the charger 2 and the developing unit 4 as shown in FIG. 1B, the charge disappears while the apparatus is at rest or standing by, although the surface of the photosensitive drum 1 in this region is charged by the charger 2 before the apparatus is brought to a rest or stand-by state. Thus, the region becomes an uncharged region (a region where the electric potential is zero).
Meantime, in the developing unit 4 that adopts the bias developing method, a developing bias potential (e.g., -350 V) which is approximately middle between the charging potential (e.g., -700 V) and the potential (e.g., about 0 V) at the exposed portion is applied to a developing roller, as shown in FIG. 2A. Thus, the toner, which has been negatively charged, electrostatically adheres to the exposed portion due to the difference between the potential of the exposed portion and the potential of the developing roller, but the toner does not adhere to the non-exposed portion because it is at the charging potential.
According to the prior art, the photosensitive drum 1, the charger 2 and the developing unit 4 are simultaneously activated in the initial sequence carried out at the time of starting the apparatus. Thus, since the developing roller of the developing unit 4 rotates with the developing bias potential applied thereto, when the above-described uncharged region of the photosensitive drum 1, which extends from the charger 2 to the developing unit 4, passes the developing unit 4, the toner adheres to the uncharged region, as shown in FIG. 2B, according to the principle as shown in FIG. 2A. Thus, the toner adheres to the whole surface of the photosensitive drum 1.
Accordingly, the toner is unnecessarily consumed, resulting in a high running cost. In addition, since the collected toner is thrown away, the prior art is unfavorable from the environmental point of view. Furthermore, the residual toner on the photosensitive drum 1 is likely to adhere to the transfer roller 5, causing the paper to be stained during the subsequent printing process.
To solve the above-described problem, a method has been proposed wherein a contact type developing unit, in which a developing roller is brought into contact with the photosensitive drum 1 to effect development, is provided with a mechanism for bringing the developing unit into and out of contact with the photosensitive drum 1, and wherein when printing is not carried out, the developing unit 4 is held separate from the photosensitive drum 1, and only when printing is to be carried out, the developing unit 4 is brought into contact with the photosensitive drum 1. The proposed method makes it possible to prevent the toner from adhering to the above-described uncharged region and hence avoid unnecessary consumption of the toner.
There has been proposed another conventional technique wherein the transfer roller 5 is provided with a mechanism for bringing it into and out of contact with the photosensitive drum 1, and wherein the transfer roller 5 has previously been separated from the photosensitive drum 1, and after the above-described uncharged region has passed the transfer roller position, the transfer roller 5 is brought into contact with the photosensitive drum 1 (for example, see Japanese Patent Application Laid-Open (KOKAI) No.02-148076). According to this method, although some toner is unnecessarily consumed, no toner will adhere to the transfer roller 5. Therefore, it is possible to prevent the transfer roller 5 from being stained with toner.
However, the conventional method in which the developing unit is selectively brought into and out of contact with the photosensitive drum suffers from the problem that since the developing unit is generally heavy in comparison to other process parts, the mechanism for bringing it into and out of contact with the photosensitive drum unavoidably becomes complicated and costly. Moreover, it becomes necessary to provide a space for the stroke of the developing unit and the mechanism therefor, resulting in an increase in the overall size of the apparatus.
The other conventional method, in which the transfer roller is withdrawn from the position where it is in contact with the photosensitive drum, can prevent the transfer roller from being stained with toner but cannot avoid adhesion of unnecessary toner to the photosensitive drum. Thus, unnecessary consumption of toner cannot be prevented.
In particular, if this method is used for a cleanerless process in which no cleaner is provided, but the residual toner is collected at the developing unit, a large amount of toner cannot satisfactorily be collected at the developing unit, so that uncollected toner is left on the photosensitive drum, causing scumming.